1. Field of the Invention
This invention relates generally to nuclear reactors and more particularly to an emergency core cooling system for a liquid metal-cooled nuclear reactor.
2. Description of the Prior Art
A nuclear reactor is designed and operated for the purpose of initiating and maintaining a nuclear fission chain reaction in a fissile material for the generation of heat for power purposes. In the type of nuclear reactor described herein, fissile materials are contained within fuel rods or elements. A plurality of fuel elements or rods comprise a fuel assembly; a plurality of these assemblies comprise a nuclear core which is structurally supported within a hermetically sealed pressure vessel. A reactor coolant, such as liquid sodium is circulated into the reactor vessel and through the nuclear core where the heat generated by nuclear fission is transferred from the fuel assemblies to the reactor coolant. The heated reactor coolant exits from the pressure vessel and flows to a heat exchanger where the heat previously acquired is transferred to another flow system coupled in sealing arrangement with the heat exchanger. The cooled liquid sodium exits from the heat exchanger and flows to a pump which again circulates the reactor coolant into the pressure vessel, repeating the described flow cycle.
The system comprising the nuclear core, reactor vessel, heat exchanger, circulating pump, and the connecting piping is commonly referred to as the primary system. Liquid metal-cooled fast breeder reactor plants characteristically have two or more primary systems or primary loops.
In a nuclear reactor, one of the many accidents which must be guarded against is a double-ended rupture of the connecting piping leading from one of the pumps to the reactor pressure vessel. A double-ended rupture is one whereby the pipe breaks in a direction generally transverse to the axial center line of the pipe. In counter distinction, this type of rupture is not along the length of the pipe such as a failure of a pipe seam. If a double-ended rupture occurs, flow will be discharged out of both ends of the pipe until the reactor is shut down and the pumps can be slowed down sufficiently so that no more coolant is being pumped through the ruptured pipes. During this time, a considerable amount of coolant that is normally supplied to the nuclear core is diverted out the ruptured pipe (from both the ruptured and the intact flow loops) and does not cool the core. This situation may cause extremely high core temperatures resulting in failure of the fuel cladding and subsequent melting of the nuclear fuel contained within the nuclear core.
In the prior art, efforts to guard against the effects of the envisioned failure have included reactor designs which include check valves in each of the main coolant flow lines as they enter the reactor vessel, or by greatly increasing the number of flow loops and thereby reducing the effect of failure of any one flow loop. Unfortunately, the optimum location for the check valve is at the bottom of the reactor pressure vessel which makes maintenance very difficult. In addition, if it is necessary to rely on check valves in this very important way, it may be necessary to provide redundancy by including a number of check valves in series in each of the main coolant flow lines. Either solution, that is, greatly increasing the number of flow loops or providing a series of check valves, is expensive and adds complications to the plant which tend to reduce the overall plant availability as regards production of commercial electrical energy.
Another prior solution is to have a hydraulic diode which has no moving parts and gives performance like a check valve in having greatly reduced flow in the backward direction included in the main coolant flow lines. Efforts have been underway for a number of years to develop a hydraulic diode to perform this function, but the efforts have not been successful. Therefore, in the prior art, no practical solution to a double-ended pipe break of a main coolant flow line has been effectuated.